The present invention relates to a communication system, a terminal device, and a communication method that perform communication based on the Real-time Transport Protocol/RTP Control Protocol (RTP/RTCP).
The terminal device, in order to establish a session with another terminal device that is subordinate to a device (a NAT router or the like, hereinafter called a NAT device) that is provided with a network address translation (NAT) function, must detect a port number that is stored in the NAT device and transmit a packet to that port number.
A known method for establishing a session between terminal devices that are subordinate to different NAT devices is described. In this method, a terminal device that is connected to a local area network (LAN) transmits a bubble packet to a communication terminal that is connected to the Internet. The port number for the LAN-connected terminal device to perform communication with the Internet-connected communication terminal is stored in the NAT device. The stored port number is transmitted through a server to a terminal device that is subordinate to a different NAT device. The terminal device that has received the port number transmits the packet using the received port number as a destination port number. The session between the terminal devices that are subordinate to the different NAT devices is thus established.